#define _CRT_SECURE_NO_WARNINGS 1
class Solution {
public:
    int numTrees(int n) {
        vector<int> dp(20);
        dp[1] = 1, dp[0] = 1, dp[2] = 2;
        for (int i = 3; i <= n; i++)
            for (int j = 1; j <= i; j++)
            {
                dp[i] += dp[j - 1] * dp[i - j];
            }
        return dp[n];
    }
};